The Viking 1 Lander sampling arm and the deep trenches it dug as part of the surface composition and biology experiments on Mars. Images credit: NASA and Roel van der Hoorn.

“This suggests a robust biological response. These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars.” -Bianciardi, Miller, Straat and Levin

On July 20, 1976, the Viking 1 lander touched down onto the Martian surface, followed just a few weeks later by Viking 2. On board both landers were a suite of three experiments designed to look for signs of life.

NASA Viking Lander Biological Experiment Package. Image credit: NASA.

While the Gas Chromatograph-Mass Spectrometer and the Gas Exchange experiment both came back negative, the Labeled Release experiment — where nutrient-rich molecules tagged with radioactive carbon-14 were added to the Martian soil — gave off a positive release of radioactive CO2.